A biaxially stretched polyamide resin film is excellent in the mechanical characteristics, barrier property, pinhole resistance, transparency, etc. and has been used widely as a wrapping material. However, due to a high hygroscopic property derived from amide bonds in the polymer structure, the mechanical strength can fluctuate, and hygroscopic elongation occurs in accordance with fluctuation in humidity and besides, problems tend to occur in many kinds of steps. Furthermore, the glass transition temperature of the resin itself is not so high and improvement of heat resistance, particularly, the mechanical properties at a high temperature, has been desired.
Moreover, a common polyamide film made of nylon 6 has a high elastic modulus but low elongation, and accordingly shows a lowered maximum point stress and rather brittle characteristics in a low humidity. On the other hand, the polyamide film has a low elastic modulus but high elongation, and accordingly shows an increased maximum point stress and ductility in a high humidity. When modifying the stretching conditions for improving the characteristics in lower humidity, there occurs a problem that the film characteristics are unbalanced. As described, a common polyamide film shows considerably changed characteristics in accordance with the humidity level as compared with a film comprising a poly(ethylene terephthalate). Therefore, it is necessary to control the humidity in the film production process and to determine the processing conditions on the basis of previous estimation of characteristic fluctuation.
Further, a biaxially stretched polyamide resin film shows a decrease of the mechanical strength and hygroscopic elongation due to the high hygroscopic property derived from amide bonds in the polymer structure. In addition, the difference of shrinkage quantities at the time of boiling the polyamide film tends to cause problems in many steps due to strains and curls formed in the film.
The strains at the time of humidity absorption and boiling are generated due to relaxation of the structure at the time of stretching. When a material with a high stretching stress is stretched, the shrinkage stress generated at the time of relaxation becomes high and the strain also becomes significant. Therefore, it is supposed to be possible that the strain or the like can be suppressed by lowering the shrinkage stress; however in the case of a polyamide resin, it becomes difficult to change the stretching stress due to the strong hydrogen bonds between molecules and thus it is difficult to lower the stretching stress. Some of previous documents disclose decrease of boiling strains; however there is no technique disclosed for lowering the stress (Reference to Patent Documents 1 to 3).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2006-96801    Patent Document 2: JP-A No. 2006-88690    Patent Document 3: JP-A No. 2007-237640
As described above, with respect to a polyamide resin film with lowered boiling strains, there is no investigation on lowering the boiling strains, while paying attention to the stretching stress.
Further, it has been known as a method for improving heat resistance and the hygroscopic property of a polyamide resin that a layered silicate can be evenly dispersed. This technique has been known well as nano-composite formation. Since the above-mentioned various characteristics can be improved by the nano-composite formation, it is expected that a film with improved characteristics can be obtained by film formation. However, in reality, the resin is generally poor in the stretching property and unsuitable as a resin for stretched films. Particularly, it is said that in order to sufficiently improve the mechanical properties, addition of 1% or more of a layered silicate to the polyamide resin is needed; however the stretching of a polyamide resin with a high layered silicate content is considerably difficult.
Patent Document 4 discloses a biaxially stretched polyamide film containing a layered silicate. The highest reaching temperature for the successive stretching in the width direction followed by the longitudinal direction for overcoming the difficulty of stretching is as high as 180 to 200° C. Not only difficulty of production, but also difficulty of crystallization, is promoted too far before sufficient stretching in the width direction is performed. As a result, there occur problems such that the layered silicate cannot be sufficiently oriented, various effects due to the addition of the layered silicate are not exhibited, thickness unevenness occurs in fine regions, and pinhole resistance cannot be satisfied.    Patent Document 4: JP-A No. 2003-20349
Further, as being understood from the scopes of claims of the specified methods described above, practically the amount of a layered compound is 1 wt. % or lower (including the organic matter contained in the interlayer). If the amount of layered compound exceeds 1 wt. %, whitening at the time of stretching and poor productivity at the time of high stretching ratio occur. These drawbacks are supposedly attributed to that the stress that tends to be converged on the tip end of the layered compound at the time of stretching such that creases and cracks are easily caused.
Further, Patent Document 5 discloses a stretched film in a system containing 0.5 to 5% of a layered inorganic compound but does not describe any concrete countermeasure for solving the above-mentioned poor stretching property nor discloses any technique for a stretching method with industrial productivity by successive biaxial stretching in a system containing a layered compound in a high concentration of 1% or higher and consequently, results in investigation in a level of simultaneous biaxial stretching of small specimens in a laboratory. Also, there is a description in the specification that a layered inorganic compound such as montmorillonite has an effect on improvement of the slipping property due to decreasing water absorption. However, an equilibrium moisture content of a resin is increased if a material such as montmorillonite is added to a nylon resin. Based on this, it can be said that the essence of the invention is significantly attributed to the effect of the addition of the inorganic lubricant.    Patent Document 5: JP-A 2003-313322
On the other hand, Nishino et al, Kobe University pointed out an interesting result with respect to evaluation of the orientation of a layered inorganic compound dispersed in a stretched film (e.g., Non-patent Document 1). In this report, they paid attention to 060 reflection in a layer of montmorillonite and reported that the reflection intensity was increased in the meridian direction by in-plane orienting montmorillonite.    Non-Patent Document 1: Molecular Nanotechnology 174th Conference, first Seminar, Preprints, p 22-23, Jul. 13, 2007
Further, with respect to piercing strength, this characteristic can be exhibited by increasing the in-plane orientation of a polyamide resin. If a layered compound is added to further heighten the piercing strength, contrarily the stretching ratio cannot be increased, and thus there is a problem that the piercing strength is scarcely improved as compared with a film containing no layered inorganic compound.
As described, in the extension of conventional techniques, the industrial production of stretched films of polyamide resin having excellent mechanical properties has been difficult.
Also, generally, for providing the slipping property to films, particles are added as a lubricant to form projections on the surfaces. However, in the case of a polyamide film, since the resin becomes soft and the slipping property is lowered due to an increase in humidity, it is required to roughen the surface for achieving the desired slipping property even under high humidity. Therefore, there occurs a problem of worsening the gloss in a polyamide film having improved slipping property under high humidity.